An infrared light filter for transmitting only an infrared light having a predetermined wavelength is, for example, a Fabri-Perot filter. The Fabri-Perot filter is disclosed, for example, in U.S. Pat. No. 6,590,710.
The fabric-Perot filter includes the first mirror as a fixed mirror and the second mirror as a movable mirror. The second mirror is disposed on the first mirror through a silicon oxide film in such a manner that the first and second mirrors face each other, and a cavity having a predetermined gap between the first and second mirrors is provided by the first and second mirrors.
When a voltage, i.e., the electric potential difference, is applied between the first and second mirrors, the second mirror is displaced or deformed by an electrostatic attractive force. Thus, the gap between the first and second mirrors is changed so that only the infrared light having a predetermine wavelength corresponding to the gap can transmit the filter.
However, the second mirror is deformed to be a balloon shape when the second mirror is displaced toward the first mirror. Therefore, the gap between the first and second mirrors becomes inhomogeneous. Accordingly, the range of wavelengths of the infrared light that the filter transmits becomes broader. Therefore, when the infrared light is detected by a detector, the detector has a large noise and low accuracy of the detection of the infrared light having a predetermined wavelength.